In the future, will we need more time to travel between galaxies?

Each galaxy is expanding away from the others. As a result, would that mean we require more time in the future to travel to other galaxies as the space in between increases as well?

Originally posted at Forbes!

It depends on how far you want to go! Generally, when an astronomer is trying to explain the idea of space expanding, they might use the idea of the space between two galaxies expanding. This is a good illustration; it communicates the idea that it’s not that the galaxies are in motion themselves, but that the space they’re embedded within is growing, so the distance between the two of them wind up growing with time.

Unfortunately, this explanation can have a bit of an unintended side effect as a metaphor- if I’m not careful, I can wind up simplifying the universe too much, so that it seems like this happens equally to all galaxies, with galaxies spaced out roughly equally from each other across the universe. But if we actually look out into the night sky, we can see that ours is not a universe of equal spacing. Ours is a universe that looks rather spidery, with filaments of the cosmic web stretching across the sky. The image at the top shows a simulation of the distribution of galaxies in our universe. There are grand areas of nothing, and clusters of thousands of galaxies, connected to other clusters with faint tendrils, each made up of glowing galaxies.

And yet, the universe is expanding, and the space between objects is increasing, due to the currently inexplicable influence of Dark Energy. Should this affect, say, the distance between the Milky Way and Andromeda?

In principle it might – but it’s important to remember two facets to the universe’s expansion. One – this expanding force is very, very small. It happens that we have an awful lot of space, so over the whole universe it’s quite significant. The second thing is that on reasonably small scales (which in this case, means objects which are the size of galaxies and smaller), gravity is much stronger.

Gravity is a bit of a juggernaut in the astrophysical world, and if two objects are gravitationally bound to each other (meaning that their relative speeds are too slow to let them escape the pull of the gravity of the other), cosmic expansion isn’t going to be able to do much about it. The expansion of the universe would have to be frighteningly enormous (much larger than we observe it to be) to pull apart two galaxies which are bound to each other by gravity. The Milky Way and Andromeda are bound, and as such, are due to collide sometime in the next 3.5 – 4.5 billion years. So if you want to go to Andromeda at any point in the future (before it collides with us), cosmic expansion won’t play a role in the time it would take to travel there. It’s not just Andromeda that we’re bound to – we have a whole cluster of galaxies that our galaxy is part of, and all of these are bound to each other gravitationally, though much more loosely than the Milky Way is to Andromeda.

But if you wanted to go beyond our cluster of galaxies, and travel intergalactic space to reach a totally separate cluster, one entirely unrelated to our galaxy – then, yes, you would need more time to travel to those galaxies in the future than you would if you left today.

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